The present invention relates to a method of operating a two-stroke-cycle engine having a variably timed exhaust valve in a four-stroke-cycle mode. More particularly, the invention relates to an additional compression stroke and an additional expansion stroke in order to enhance vaporization of low vapor pressure fuels in a cold engine.
Internal combustion engine valves in four-stroke cycle engines are almost universally of a poppet type which are spring loaded toward a valve-closed position and opened against that spring bias by a cam on a rotating cam shaft with the cam shaft being synchronized with the engine crankshaft to achieve opening and closing at fixed preferred times in the engine cycle. This fixed timing is a compromise between the timing best suited for high engine speed and the timing best suited to lower speeds or engine idling speed. The valves in two-stroke-cycle engines are generally simple apertures or ports in the cylinder sidewall which are uncovered or opened by piston movement, however, exhaust valving of the cam actuated as well as other varieties have been suggested.
A two-stroke-cycle compression ignited (Diesel) engine utilizing a conventional cam actuated overhead valve as the exhaust valve with the traditional cylinder sidewall intake ports receiving pressurized scavenging air from a positive displacement (Roots) blower is known. The exhaust valving of this known Diesel engine suffers from the above defects, but when operated over a narrow range of speeds, it operates with relatively high efficiency since there are little or no throttling losses in its operation. It would be highly desirable to be able to operate a spark-ignited two-stroke-cycle engine over a wide range of speeds with little or no throttling losses, but up until now this has not been possible because such spark-ignited engines require a fuel to air ratio mix versus retained exhaust gas within a fairly narrow range of values for successful ignition. Control of such an engine, then, requires some measure of control over both the quantity of fuel entering the cylinder and a control of the quantity of air entering the cylinder as well as the quantity of retained exhaust gas. The control of the quantity of air entering the cylinder has, up until now, been controlled by a restriction or throttling of the air path into the cylinder against which piston motion had to work to suck the desired quantity of air into the cylinder. Such throttling has been so commonplace that the traditional name attached to the engine speed control in aircraft, boats, steam engines and many other craft is "throttle".
A two-stroke-cycle spark-ignited engine utilizing a conventional ignition system and having a fuel injector which introduces a controlled quantity of fuel directly into the closed end of the cylinder cavity has also been proposed. This engine utilizes the traditional cylinder sidewall intake ports receiving pressurized scavenging air from a positive displacement blower and cylinder sidewall exhaust ports which, in addition to being opened and closed by piston travel, are valved by rotary exhaust valves. The exhaust valving of this known Otto cycle engine appear to be either chain or cam driven, but, in either case, appears to be fixed in its timing.
The prior art has recognized numerous advantages which might be achieved by replacing such cam actuated or similar valve arrangements with other types of valve opening mechanism which could be controlled in their opening and closing as a function of engine speed as well as engine crankshaft angular position or other engine parameters.
U.S. Pat. No. 4,945,870 entitled Vehicle Management Computer discloses a computer control system which receives a plurality of engine operation sensor inputs and in turn controls a plurality of engine operating parameters including ignition timing and the time in each cycle of the opening and closing of the intake and exhaust valves among others. This patent teaches numerous operating modes or cycles in addition to the conventional four-stroke cycle. In particular, this patent discloses the principles suitable for implementing a control computer for the method of the present invention.
U.S. Pat. No. 4,878,464 entitled Pneumatic Bistable Electronic Valve Actuator, discloses a valve actuating device which is a jointly pneumatically and electromagnetically powered valve with high pressure air supply and control valving to use the air for both damping and as one motive force. A magnetic motive force is supplied from the magnetic latch opposite the one being released and this magnetic force attracts an armature of the device so long as the magnetic field of the first latch is in its reduced state. As the armature closes on the opposite latch, the magnetic attraction increases and overpowers that of the first latch regardless of whether it remains in the reduced state or not. This patent also discloses different operating modes including delayed intake valve closure and a six stroke cycle mode of operation.
U.S. Pat. No. 4,875,441 entitled Enhanced Efficiency Valve Activator discloses a further valve actuator which would be suitable in implementing the method of the present invention. This patent further includes a good summary of the state of the art in valve actuators at the time the application was filed.
U.S. Pat. Nos. 4,945,870, 4,878,464 and 4,875,441 are specifically incorporated herein by reference. These patents are all assigned to the assignee of the present invention.
As suggested in U.S. Pat. Nos. 4,878,464 and 4,945,870, a four-stroke-cycle engine having variably timed intake and exhaust valves could be operated in a six-stroke-cycle mode. The additional essentially adiabatic compression and expansion strokes more thoroughly evaporating the fuel and mixing the fuel and air. This permits burning low vapor pressure fuels in a four-stroke-cycle spark ignited engine under cold start conditions.
Automotive fuel, by evaporation, is a major contributor to atmospheric hydrocarbons which cause smog. The major sources of evaporative emissions are carburetors, fuel tanks, storage and transfer facilities at local outlets (gas stations), and major distribution systems. The evaporation problem is exacerbated by the use of high vapor pressure gasoline having highly volatile constituents which make it possible to start spark ignited engines over a wide temperature range.
FIG. 1 shows the effect of temperature on the vapor pressure of a typical modern gasoline; at present this pressure runs 9-13 psig at 100.degree. F. FIG. 2 shows the effect of temperature on the vapor pressure of isooctane, which is the major constituent of gasoline without volatile additives. The vapor pressure of isooctane is only about 1.7 psig at 100.degree. F. FIG. 3 shows the vapor pressure of paraffins and olefins at 100.degree. F. versus number of carbon atoms per molecule. Butane (C.sub.4 H.sub.10) and heptane (C.sub.7 H.sub.16) are typical paraffins added to gasoline to increase the vapor pressure. FIG. 4 is a plot of experimentally measured fuel tank evaporative emissions versus Reid vapor pressure (RVP) of the gasoline in a 25 gallon tank containing ten gallons of gasoline. If the RVP is reduced from 10 to 5 psig, the fuel loss due to evaporative emissions is reduced from 70 g to 5 g, or about 1400%. However, starting problems at low ambient or equipment temperatures become progressively more serious as the RVP is reduced.